Aluminizing of ferrous metal base



United States Patent 3,047,420 ALUMINIZING F FERRUS METAL BASE Leslie M. Bernick, Calumet City, and Henry M. Roelofs,

Chicago, Ill., assignors to Inland Steel Company, Chicago, Ill., a Delaware corporation Filed Nov. 3, 1958, Ser. No. 771,287 Claims. (Cl. 117-51) This invention relates to improvements in the coating of a ferrous metal base with aluminum. More particularly, the invention relates to improvements in the pretreatment of a ferrous metal -base prior to hot dip aluminizing.

It has long been known that aluminum can be applied to the surface of steel or other ferrous metal to improve corrosion resistance and high temperature scaling resistance. In many respects, an aluminum coating has significant advantages over zinc as in the widely used galvanized coatings. Although aluminum can be applied to a ferrous metal base such as steel by various methods, the hot dip method wherein the ferrous metal base is immersed in a bath of molten aluminum appears to be the most economical and practical where it is desired to provide a coating thickness of the same order of magnitude as the zinc coating in conventional galvanized steel. However, in spite of the excellent corrosion and oxidation resistance of aluminum coatings, the use of hot dip aluminum coatings has been limited heretofore because of the apparent inability to provide a product having both good corrosion resistance and also good formability characteristics. By the term formability is meant the ability of the aluminum coated base to withstand bending or deformation during working and drawing operations without the coating cracking or becoming detached from the base.

It is known that the principal factor determining formability or ductility of the aluminum coating is the thickness of the brittle interface iron-aluminum alloy layer which forms rapidly at the elevated temperature of the hot dip coating step. r[he thickness of the interface alloy layer is governed by several variables including the composition of the coating bath, the temperature of the bath, the thickness of the ferrous metal base being coated, and the length of time during which the base is immersed in the molten aluminum bath. To obtain acceptable formability of the aluminized product, it is generally considered that the thickness of the interface iron-aluminum alloy layer should not exceed about .0004 in.

Previous efforts to minimize or retard the formation of the interface alloy layer have involved several different approaches. Careful control over the time and temperature of the hot dip coating step is capable of providing some improvement but does not represent a complete solution to the problem. It is also known to include in the molten aluminum bath added amounts of other metals such as silicon, beryllium, or copper which retard interface alloy formation. Although the aluminum coated product thus obtained may have acceptable formability, its corrosion resistance is seriously diminished. Another approach to the problem involves the use of a mechanical or superficial barrier metal plated on the ferrous metal base prior to aluminizing in order to restrict interface alloy formation. Satisfactory results have been reported using barrier metals or alloys such as gold, silver, copper, cobalt, nickel, cadmium, zinc, tin, mercury, molybdenum, and tungsten. However, even if an acceptable aluminized product having good corrosion resistance and good formability is obtainable by the barrier technique, nevertheless, the cost of the dual coating operation would be prohibitive from a practical viewpoint.

A principal object of the present invention is to pro- 3,047,420 Patented July 31, 1962 rice vide novel improvements in the hot dip aluminizing of a ferrous metal base such that the aluminum coated product has good corrosion resistance and good formability.

An additional object of the invention is to provide an improved process of the foregoing character which is economically feasible and is particularly adapted for use in a continuous line.

A further object of the invention is to provide a novel and improved method of minimizing interface iron-aluminum alloy formation during the hot dip aluminum coating of a ferrous metal base.

Another object of the invention is to provide an improved method of utilizing molybdenum or tungsten to retard the formation of interface iron-aluminum layer during the hot dip aluminum coating of a ferrous metal base.

Other objects and advantages of the invention will become apparent from the subsequent detailed description taken in conjunction with the accompanying drawing, wherein:

FG. l comprises a schematic flow sheet illustrating one specific embodiment of the invention; and

FlG. 2 comprises a modification of the process of FIG. l.

As mentioned above, it has been suggested in the prior art to apply mechanical or superficial barrier layers of various metals to the steel or other ferrous metal base prior to the hot dip aluminizing step. For example, in US. Patent No. 2,800,707 it is proposed to apply molybdenum or tungsten as a superficial barrier layer at the surface of the ferrous metal base so as to retard the extent of interpenetration of iron and aluminum at the interface. However, the present invention is based on our discovery that a mere superficial coating of molybdenum or tungsten is not always capable of yielding an aluminized product having good corrosion resistance and acceptable formability. On the contrary, we have now found that the most practical and convenient way to apply molybdenum or tungsten in a continuous operation is by reduction of molybdenum oxide or tungsten oxide and that a superficial barrier layer formed in this manner does not give the desired properties in the aluminized product. Instead, consistently good results are obtained only if the molybdenum or tungsten obtained by reduction of their oxides is completely diffused into the surface of the ferrous metal base at an elevated temperature and other necessary precautions are taken to insure that there is no free or undiffused molybdenum or tungsten on the surface of the base.

Briefly described, the improved aluminizing process of f the present invention comprises the steps of (l) providing on the surface of the ferrous metal base an initial oxide coating of either molybdenum oxide (M003) or tungsten oxide (W03) or mixtures thereof, (2) heating the oxide coated base in a reducing atmosphere at a temperature of from about l850 F. to about 2l00 F. for a period of time sufficient to effect reduction of the oxide to the corresponding metal and also sufficient to obtain complete diffusion of the reduced molybdenum or tungsten into the surface of the base, and (3) immersing the base in a molten aluminum bath. As will hereinafter appear, in order to obtain the desired degree of control so that the thickness of the interface alloy layer does not exceed .0004 in., the amount of diffused molybdenum or tungsten in the base must be at least about 0.3 gm. per sq. ft. of surface, preferably from about 0.3 to about 1.0 gm. per sq. ft.

In the oxide coating step of the process, molybdenum oxide (M003) or tungsten oxide (W03) may be deposited as such formed in situ on thev surface of the ferrousmetal base. ing the oxide coating on the surface of the base is to One convenient method of provid-- f2 form a suspension or dispersion in water or other suitable liquid medium of the molybdenum or tungsten oxide in finely divided or powdered form and then apply the liquid coating material to the surface of the base by any suitable technique such as mechanical spraying, electrostatic spraying, brushing, roll coating, dipping, etc. Usually, water will be the most convenient liquid medium but other liquid vehicles such as glyccrine, light oils, etc. may be used. In the case of an aqueous medium, it is frequently advantageous to employ an added dispersing or suspending agent, such as starch or various surface active agents, in order to form a relatively stable suspension or dispersion of the finely divided oxide in the aqueous liquid. We have found that corn starch is highly satisfactory and economical 4for this purpose. A wetting agent or surface tension-reducing agent may lalso be added to yfacilitate wetting of the base, especially where the latter has an oil film.

It is also within the scope of the invention to provide the molybdenum or tungsten oxide coating on the surface of the ferrous metal base by means of a solution or suspension of a compound or salt which can be decomposed or oxidized to yield the desired oxide in situ. For example, ammonium molybdate or molybdic acid can be deposited on the metal surface and are readily decomposable by heating to form the desired molybdenum trioxide. Similarly, ammonium tungstate, sodium tungstate, or tungstic acid can be applied to the metal surface and decomposed by heating to form the desired tungsten trioxide.

As heretofore mentioned, for satisfactory formability of aluminized steel it is generally considered that the thickness of the interface iron-aluminum allloy layer should not exceed .0004 in. with an aluminum coating three to five times the thickness of the alloy layer. Our investigations have shown that in order to restrict interface alloy formation to this extent in accordance with the pnocess of he present invention, it is necessary to provide during the oxide coating step a sufficient amount of molybdenum oxide or tungsten oxide so that upon subsequent reduction and diffusion thereof there will be obtained not less than about 0.3 gm. of diffused molybdenum or tungsten per sq. ft. of surface. Increasing the amount of `diffused molybdenum or tungsten gives improved results up to about 1.0 gm. per sq. ft. at which point the thickness lof the interface iron-aluminum alloy layer is reduced to about .0001 in. or less. Although larger amounts of diffused molybdenum or tungsten may be used, there appears to be no particular advantage which would justify the use of such increased amounts as far as `any improvement in formability of the final aluminized product is concerned. Accordingly, in the preferred manner of practicing the invention the contained molybdenum or tungsten in the coating of oxide or compound decomposable to the oxide should be such as to yield from about 01:3 to about 1.0 gm. of diffused molybdenum or tungsten per sq. ft. of surface.

Following the application of the molybdenum or tungsten oxide coating on the surface of the ferrous metal base, the next essential step of the invention is to heat the oxide coated base to an elevated temperature in a reducing atmosphere to effect rapid reduction of the molybdenum or tungsten oxide to the corresponding free molybdenum or tungsten and also to cause complete diffusion of the reduced molybdenum or tungsten into the ferrous metal base. It will be understood that when a decomposable compound yof Imolybdenum or tungsten is used instead of the oxide, the desired thermal decomposition to the oxide and reduction of the latter are accomplished in a single operational step. The reductiondiffusion effects are dependent upon temperature, time, and the hydrogen concentration of the reducing atmosphere. However, the temperature of the reduction-diffusion step is the most critical aspect of the process. Whereas it has been suggested in the prior ant that the presence of free molybdenum or tungsten as a superficial barrier layer at the surface of the ferrous metal base is effective to obtain the desired control over interface alloy formation, we have found that in the case of molybdenum or tungsten obtained by reduction of the corresponding ltrioxides it is essential that all of the molybdenum or tungsten be completely diffused into the ferrous metal base so that the surface is substantially free of undiffused molybdenum or tungsten. Although reduction of molybdenum or tungsten trioxides to the corresponding free metal may be accomplished at a temperature as low as about 1700 F. in a hydrogen atmosphere, nevertheless, in :order to obtain the required complete diffusion of the reduced molybdenum or tungsten in any feasible period of time the reduction-diffusion step of the present invention Lmust be carried out at a temperature of not less than about 1850 F. and preferably not less than about 1950 F. As a practical matter, the upper limit Iof temperature for the reduction-diffusion step is about 2100 F. but there is no special advantage in operating much above about 1950D F.

With a reduction-diffusion temperature of 1850-2l00 F. it is possible to realize the desired reduction and diffusion within a time period of from about `1/2 to about 5 min. when using a reducing atmosphere containing from about 20% to about 100% hydrogen. For a continuous operation with a line speed ranging from about 50 to about 250 `ft. per min., a temperature of 1950-2100 F. and a hydrogen concentration of from about 50% to about 100% are preferred so as to obtain the desired reduction-diffusion in a matter of from about 1 to about 3 min. A longer time at diffusion temperatures of 1950- 2100 F. increases the depth of the diffusion layer and adversely affects the distribution of molybdenum or tungsten in the diffusion layer. An atmosphere of dissociated ammonia comprising about hydrogen and about 25% nitrogen is particularly useful. A reducing atmosphere of lower hydrogen content, such as the commercially used HNX atmosphere containing 5 to 12% hydrogen, could also be used but this necessitates an increased residence time in the reduction-diffusion zone which is generally undesirable because of the excessively long line which would be required for continuous operation of the process.

As just described, in the preferred embodiment of the invention the reduction and diffusion effects are accomplished in a single heating step or zone at the conditions specified. However, it is also within the scope of the invention to conduct the reduction and diffusion in more or less separate and distinct stages. For example, the reduction step could be effected in a first Zone with a reducing atmosphere and, if desired., a temperature somewhat below t-he optimum diffusion temperature range of 1S50-2100 F. Thereafter, the diffusion of the reduced molybdenum or tungsten could be accomplished in a second zone at a temperature of 1850-2100 F., preferably 1950-2100" F., and in a non-oxidizing atmosphere which may be either reducing or neutral in character.

The ferrous metal base having its surface diffused with molybdenum or tungsten in the manner just described is then immersed in a molten aluminum bath to provide the desired aluminum coating in accordance with the well known hot dip coating method. In general, the molten aluminum bath is maintained at a temperature of from about 1200 F. to about 1300 F. or higher and the immersion time may be from about 2 to about 10 sec. to obtain an aluminum coating ranging from about .0001 to about .003 in. in thickness. For optimum corrosion resistance, a substantially pure aluminum coating should be used but itis also within the scope of the invention to employ a molten aluminum bath containing various alloy additions. It is to be understood that the ter-m molten aluminum bath as used herein is inclusive of the various aluminum alloy baths.

As `will be evident from the experimental data hereinafter set forth, the benefits and advantages of the invention are realized only when the reduction-diffusion step is carried out under the conditions heretofore specified so as to insure complete diffusion of the reduced molybdenum or tungsten into the ferrous metal base. Furthermore, the amount of diffused molybdenum or tungsten must be at least 0.3 gm. per sq. ft. of surface in order to limit the interface iron-aluminum alloy layer formed during the subsequent hot dip aluminizing step to a maximum thickness of about .0004 in. and thereby realize optimum formability characteristics in the final aluminum coated product. The surface of a ferrous metal base prepared in accordance with the present invention wherein the molybdenum or tungsten is completely diffused has a light gray color with the appearance of a tight integral deposit. On the other hand if the reduced molybdenum or tungsten is not completely diffused the coating on the base is loose or powdery and has a dark gray color. When the reduced molybdenum or tungsten is diffused into the base it becomes alloyed with, i.e. forms a solid solution in or an intermetalic compound with, the iron of the base. The amount of molybdenum or tungsten in the diffusion alloy does not exceed about 18% by wt. so that the molybdenum or tungsen diffused ferrous metal base is sharply distinguished from a base having a supericial barrier coating of substantially pure molybdenum or tungsten. As heretofore mentioned the depth of the diffusion layer should not be exces-sive but in most instances should be at least about .0001 in. and preferably from about .0002 to about .0004 in. in thickness. The 'hardness of the diffused layer is somewhat less than the hardness of the base in the case of a lsteel base. This is probably a result of diffusion of carbon inwardly during diffusion of molybdenum or tungsten into the surface of the base. Because of its relative softness, the molybdenum or tungsten diffused zone has good ductility and no difficulties are encountered during bending.

If, in practicing the present invention, the reduced molybdenum or tungsten is not fully diffused into the ferrous metal base but remains wholly or partly on the surface as a superficial coating of free molybdenum or tungsten, satisfactory results are not obtained upon subsequent coating with Imolten aluminum. It is believed that there are several reasons for such poor results. In the fir-st place, an undiffused deposit of molybdenum or tungsten obtained by chemical reduction of the corresponding trioxide is in the form of a loose powder which tends to contaminate the aluminum coating. Such inclusions provide definite planes of weakness in the aluminum coating and seriously impair the integrity and formability of the coated product. In addition, an undiffused coating of molybdenum or tungsten obtained by reduction of the corresponding trioxide has only a limited effect in retarding interface iron-aluminum alloy formation, whereas lif the molybdenum or tungsten is completely diffused into and alloyed with the iron of the base in accordance with the present invention, the thickness of the interface alloy layer can be limited to as little as .0001 in. or less. Furthermore, the presence of even a slight amount of undiffused molybdenum or tungsten as a loose powdery surface layer seriously interferes with proper wetting action of the molten aluminum on the surface of the base. As is well known, in any hot dip coating technique a clean surface is indispensable to proper wetting by the molten coating metal. Accordingly, in the present invention it is impossible to `obtain a uniform adherent coating of aluminum unless`the surface of the ferrous metal base is completely free of undiff-used molybdenum or tungsten.

Because of the critical importance of obtaining a surface completely free of reduced but undiffused molybdenum or tungsten, it is also within the scope of the invention to utilize a cleaning step following the reductiondiffusion step and prior to the hot dip aluminizing step so as to insure the removal of any excess undilfused molybdenum or tungstem. For example, it may sometimes happen, particularly at the higher concentrations of molybdenum or tungstem, that sufficient molybdenum or tungsten is diffused into the base to meet the requirements for retarding interface lalloy formation but there still remains an excess of reduced molybdenum or tungsten on the surface which does not become fully diffused. In order to correct this condition prior to hot dip coating with aluminum, the excess undiffused molybdenum Or tungsten -must be removed by any of several different cleaning techniques. According to one convenient method the removal may be accomplished electrolytically by anodic cleaning in an alkaline solution, e.g. in dilute aqueous caustic soda at a current -density of about 100 amp. per sq. ft. for a period of about l0 sec. The undiffused molybdenum or tungsten may also be removed by chemical treatment with a suitable reagent capable of dissolving free molybdenum or tungsten. Mechanical methods of removal may also be employed, e.g. abrasive cleaning by sand blasting, wire brushing, or the like, or ultrasonic cleaning.

Although the process of the invention can be conducted as a batch operation or on a discontinuous or semi-continuous basis, it has particular utility and commercial attractiveness when carried out as a continuous operation for aluminizing a ferrous 4metal strip, sheet, strand or wire. The process is applicable to the coating of ferrous metal bases generally, but particularly including cold reduced plain carbon steels of low, medium, or high carbon content and also alloy steels.

Referring now to the drawing, FIG. l is a schematic representation of a preferred manner of practicing the invention on a lcontinuous basis as applied to cold reduced steel strip. The strip l0 is `fed from a payoff coil 11 through a looping tower 12 and thence to a cleaning step. Since the cold reduced steel strip normally carries an oily residue or film, it is desirable to remove the oil residue prior to aluminizing. Moreover, different lots of steel will Vary in surface cleanliness, and a cleaning step is desirable to insure a uniform continuous diffusion of molybdenum or tungsten. lf the molybdenum or tungsten diffusion is non-uniform or discontinuous, there will be a marked tendency for excessive interface alloy penetration at the thin spots or discontinuities. Any convenient cleaning method may be employed, e.g. the oil may be removed by washing with an alkaline cleaning solution or fby degreasing with a suitable liquid or vapor solvent, or the oil residue can be burned off by heating the strip in an oxidizing atmosphere. However, an electrolytic cleaning step is particularly useful. Thus, the strip 10 is passed through an electrolytic bath 13 which may comprise an alkaline solution and an electric current is passed through the strip. Such eleotrolytic cleaning operations are well known in the art.

The cleaned strip passes through a rinse zone 14 and thence upwardly to an elevated position from which it passes downwardly through an electrostatic spraying step 15. In this step the opposite sides of the strip receive a uniform coating of an aqueous suspension of molybdenum trioxide or tungsten trioxide or an aqueous solution of a soluble molybdenum or tungsten compound capable of being decomposed to the oxide as previously described. Although a preliminary cleaning step 13 is preferred, it is also possible to burn off the -oily residue at this point, i.e. after the application of the oxide cOating, and this expedient may be particularly useful in adapting the process to existing equipment. The coated strip then passes through a reduction-diffusion zone 16 in which the strip is heated to an elevated temperature of about 1950 F., in a reducing atmosphere comprising dissociated ammonia and containing about 75% hydrogen. Complete reduction and diffusion are obtained in about 2 minutes. As previously described, the amount of oxide applied in zone 15 and reduced and diffused in zone 16 is such as to provide at least about 0.3 gm. of diffused molybdenum or tungsten per sq. ft. of surface.

From the reduction-diffusion zone 16 the strip passes beneath the surface of a molten aluminum bath 17, around a sinker `roll 18, and thence upwardly to a rewind coil 19.

In FIG. 2 a modification of the continuous operation is illustrated wherein the strip as it is dicharged from zone passes through a reduction-diffusion zone 20 and thence to an anodic cleaning step 21 wherein any excess undiffused free molybdenum or tungsten on the surface of the strip is removed. The strip is then passed through an acid pickling bath 22 to remove oxides and thence through a rinse 23. Thereafter, the diffused and surface cleaned strip is immersed in a molten aluminum bath 2.4. The interposition of the anodic cleaning step 21 between the reduction-diffusion step and the hot dip aluminum coating step 24 prevents any possibility of contamination of the aluminum coating with molybdenum or tungsten and thereby avoids impairment of the ductility of the aluminum coating by reason of inclusions of free molybdenum or tungsten, as previously described.

In order to illustrate certain features of the invention the following non-limiting examples are presented.

Example I A series of tests were carried out to investigate the most 'appropriate conditions for effecting reduction of molybdenum oxide and diffusion of the reduced molybdenum in a single heating step. The test specimens comprised a cold reduced low carbon steel having the following composition on a weight percent basis: 0.07% C, 0.35% Mn, 0.01% P, 0.028% S, and 0.009% Si.

The test panels were first cleaned by means of an alkaline cleaner or by Vapor degreasing in order to remove the residual oil film. The oil free test panels were then given an oxide coating by immersing the panels for a brief time in an aqueous starch-containing suspension of molybdenum trioxide (M003). The suspension contained gm. per liter of corn starch and from about 60 to about 130 gm. per liter of molybdenum trioxide. After immersion the test panels were allowed to drain and dry, the amount of the oxide coating varying from about 0.3 to about 1.0 gm. of contained molybdenum per sq. ft. of panel surface. The oxide coated panels were then heated in an atmosphere comprising 75% hydrogen and 25% nitrogen at Various temperatures and times with the following results:

Reduction and Ditlusion Temperature F.)

Time at Temp. (Min.)

Appearance of Coating after Treatment black and dark gray, loose. darkDgray, loose.

Example Il The procedure described in Example I was followed with the exception that instead of an aqueous starchcontaining suspension of molybdenum troxide, a saturated aqueous solution of ammonium molybdate was ernployed containing about 0.1% of a non-ionic wetting agent comprising an alkylphenol-ethylene oxide condensate. Similar results were obtained.

Example III Test panels were treated in accordance with the procedure described in Example I to provide an amount of diffused molybdenum ranging from 0.07 to 1.5 gm. per sq. ft. of surface. The reduction-diffusion step was carried out in an atmosphere of 75 hydrogen-25% nitrogen at 1950 F. for two minutes in each case. The samples were aluminum coated by immersing them in a molten bath of substantially pure aluminum maintained at about 1280 F. The molybdenum diffused panels were preheated to approximately the temperature of the aluminum bath prior to immersion therein.

The effects of variation in the amount of diffused molybdenum were studied by evaluating the aluminized samples in two Ways:

(1) The ability of the sample to `withstand a lock forming operation was judged by the number and depth of cracks present when the test sample was bent 180 `on its own thickness, the results being designated as poor, fair, good, or excellent.

(2) The test samples both before and after the bend test were examined mi-croscopioally to determine the eX- tent of interface iron-aluminum alloy formation and the mechanism of cracking of the unsatisfactory samples.

The results of the tests were as follows:

As will be evident from the foregoing data, restriction of the formation of interface alloy to a maximum thickness of .0004 in., which is `considered to be necessary for satisfactory formability, was obtained with a diffused molybdenum content of .3 gm. per sq. feet. or higher. The results with the diffused molybdenum content of `0.3 gm. per sq. ft. compared favorably with the formability and the interface alloy formation of a commercially `available aluminized steel having an aluminumsilicon alloy coating. However, as shown by the test data, larger amounts of molybdenum give a more effective reduction of the interface alloy layer, e.g. down to .0001 in., with excellent formability. Nevertheless, the data do not indicate any advantage lin exceeding 1.0 gm. of diffused molybdenum per sq. ft. of surface.

We claim:

l. A continuous process for aluminum coating a ferrous metal base in strip, sheet, strand, or wire form which comprises -coati-ng the .surface of the base with a liquid containing a compound selected from the group consisting of molybdenum trioxide, -tungsten trioxide, and compounds decomposable to said oxides, the amount of said compound being sufficient, upon subsequent reduction and diffusion thereof, to yield at least about 0.3 gm. per sq. ft. of surface of a metal selected from the group consisting of molybdenum and tungsten; passing the coated base through a reduction-diffusion zone containing a reducing atmosphere and therein heating the coated base to a temperalture of at least about 1850 F. for a period of ltime suflicient to effect `reduction of the oxide to said metal and complete diffusion of the metal into the base, the diffused portion of the base having a thickness of at least about .0001 and comprising not more than about 18 wt. of said metal; 'and thereafter immersing the base in a molten aluminum bath.

2. The process of claim 1 further characterized in that the thickness of said diffused portion ot the base is from about .0002 to about .0004 in.

3. The process of claim 1 further characterized in that the diffused metal is in the form of Ian alloy with the iron of said base.

4, The process of claim l further characterized in that the amount of said compound is such as to yield from about 0.3 to about 1.0 gm. of `diffused metal per sq. tt. ot surface.

5. The process :of claim 1 further characterized in that said temperature is from about 1850 F. to about 2100 F.

6. The process of claim 1 further characterized in that said temperature is from about 19501o F. to about 2100 F.

7. The process of claim l further characterized in that the amount of said compound is such as to yield from about 0.3 to about 1.0 gm. of diffused metal per sq. ft. of surface, said reducing atmosphere contains from about 20% to about 100% hydrogen, said temperature is from about 1850 F. to about 2100 F., and said time is from about 1/2 to about 5 minutes.

8. The process of claim 1 unther characterized in that the amount of said compound is such as to yield from about 0.3 to about 1.0 gm. of diffused metal per sq. ft. of surface, said reducing atmosphere contains from about 50% to about 100% hydrogen, said temperature is from about 1950 F. tto about 2100 F., and said time is from about 1 to about 3 minutes.

9. The process of claim -1 `further characterized in that said coating step is effected -by applying to the base an aqueous suspension of finely divided molybdenum trioxide.

10. The process of claim 9 further characterized in -that said suspension also contains starch.

1l. The process ot claim 1 further characterized in that said coating step is effected by applying to the base an aqueous solution of a Water soluble compound of molybdenum, said compound being decomposed -to molybdenum tnioxide during said heating in the reducing atmosphere.

12. The process of claim 11 further characterized in that said compound comprises a molybdate.

13. An atrticle made in accordance with the process of claim 1.

14. A continuous process for aluminum coating a fernous metal base in strip, sheet, Strand, or Wire form which comprises coating the surface of the base with a liquid containing a compound selected from :the group consisting of molybdenum tr-ioxide, tungsten trioxide, and compounds decomposable to said oxides, the amount of said compound being sufficient, `upon subsequent reduction and `diffusion thereof, to yield at least about 0.3 gm. per sq. ft. of a metal selected firom the group consisting of molybdenum and tungsten; passing the coated base through a reduction-diffusion zone containing a reducing atmosphere and therein heating the coa-ted base to a temperature of at least about 1850 F. for Va period of time sufficient to effect reduction of the Oxide to said metal and complete diffusion of the metal into the base, the diffused portion of the base having a thickness of at least about .0001 in. and comprising not more than `about 18 Wt. of said metal; removing from the surface of the base any undiffused free metal; and thereafter immersing the base in a molten aluminum bath.

15. The process of claim 14 further characterized in that removal of undiffused free metal from the surface of said base is effected by anodic cleaning.

References Cited in the file of this patent UNITED STATES PATENTS 2,512,141 Ma et al June 20, 1950 2,583,163 Wasserman Jan. 22, 1952 2,793,423 Stumbock May 28, 1957 2,800,707 Whitfield July 30, 1957 2,823,139 Schulze Feb. 1l, 1958 2,865,088 Yntema Dec. 23, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,047,420 July 31, 1962 Leslie M., Bernick et al.

It is hereby certified that error appears in the above numbered petent requiring correction and that the said Letters Patent should reed as corrected below.

Column 2, line 70, after "such" insert or column 4, line 67, for "OOOF' read .O01

Signed and sealed this 15th day of January 1963.

(SEAL) Attest:

ERNEST w. swlDER DAVID L. LADD Aneting Officl Commissioner of Patents 

1. A CONTINUOUS PROCESS FOR ALUMINUM COATING A FERROUS METAL BASE IN A STRIP, SHEET, STRAND, OR WIRE FORM WHICH COMPRISES COATING THE SURFACE OF THE BASE WITH A LIQUID CONTAINING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM TRIOXIDE, TUNGSTEN TRIOXIDE, AND COMPOUNDS DECOMPOSABLE TO SAID OXIDES, THE AMOUNT OF SAID COMPOUND BEING SUFFICIENT, UPON SUBSEQUENT REDUCTION AND DIFFUSION THEREOF, TO YIELD AT LEAST ABOUT 0.3 GM. PER SQ.FT. OF SURFACE OF A METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBEDNUM AND TUNGSTEN; PASSING THE COATED BASE THROUGH A REDUCTION-DIFFUSION ZONE CONTAINING A REDUCING ATMOSPHERE AND THEREIN HEATING THE COATED BASE TO A TEMPERATURE OF AT LEAST ABOUT 1850*F. FOR A PERIOD OF TIME SUFFICIENT TO EFFECT REDUCTION OF TEH OXIDE TO SAID METAL AND COMPLETE DIFFUSION OF THE METAL INTO THE BASE, THE DIFFUSED PORTION OF THE BASE HAVING A THICKNESS OF AT LEAST ABOUT .0001 IN. AND COMPRISING NOT MORE THAN ABOUT 18 WT. % OF SAID METAL; AND THEREAFTER IMMERSING THE BASE IN A MOLTEN ALUMINUM BATH. 